66 PANAMIC DEEP SEA ECHINI. 



median edge of each plate, thus forming an irregular verticil median 

 line of two rows of miliaries. In the outer edge of the inferambulacral 

 plates there are only two or three small miliaries at the junction of the 

 ambulacral and interambulacral plates. The primary interambulacral tuber- 

 cles are perforate and crenulate (PI. 29, figs. ..'. ;). 



In a smaller specimen 12 mm. in diameter and 7 mm. in height the 

 apical system was 6.2 mm. in diameter (PI. 27, fig. 4), the anal system 4 mm., 

 and the actinal system 4.5 mm. It had six and seven primary interam- 

 bulacral tubercles with three ambulacral plates to each of the largest 

 coronal plates. The longest radiole was 16 mm. 



In a smaller specimen, 10.5 mm. in diameter and 6 mm. in height, there 

 are already eight and eight interambulacral plates, Fig. 98, as many as in 

 older specimens of nearly twice the size, though the primary tubercles 

 are only developed to their full size on the coronal plates above the ambitus 

 (PI. 29, fig. ,.'). There are three ambulacral plates to the larger coronal 

 plates. In the median interambulacral belt the miliaries are few in number, 

 forming almost a single, angular, vertical line along the median zone. On 

 the outer edge only two to three miliaries are found in the space separating 

 the scrobicular area from the ambulacra. 



t&5 &^) 



7.3 mm. 10.5 mm. 7,3 mm. 



Fig. 99. D. horridum. Fig. 100. Fig. 101. D. hobbidttm. 



In the younger specimens examined (PI. 27, figs. 1-4) the genital and 

 ocular plates are nearly of the same height, the ocular plates gradually 

 becoming more elongate than the genitals, Figs. 99, 100. 



In the smallest specimen collected, 6 mm. in diameter and 4 mm. in 

 height, there are five and six primary tubercles and three ambulacral plates 

 to each of the larger interambulacral plates. Usually there is only a single 

 miliary to each ambulacral plate ; the median interambulacral miliaries are 

 limited to a cluster of two or three in the median angle of each plate. 



The apical system (PI. 27, figs. /, 2) measured 4 mm. in diameter, the 

 anal 2.75. The plates of the genital ring (PI. 27, fig. /) are higher in 



